The presence of liquid water in a vacuum chamber can be detrimental to many vacuum processes. This is particularly the case for gas-phase chemical sterilization chambers as the presence of liquid water can inhibit sterilization at surface locations where the liquid water is located.
In practice, items to be sterilized (the sterilization load) should be thoroughly cleaned and dried prior to sterilization. A chemical gas-phase sterilization process proceeds as follows: the sterilization load is placed into a vacuum chamber and a sealing door is closed; a vacuum pump removes the atmosphere within the chamber to a medium vacuum level; the chamber is closed from the vacuum pump with a valve and a chemical gas or vapor sterilant in added to the chamber; the low pressure in the chamber allows the sterilant gas to flow and diffuse throughout the sterilization load, killing spores, viruses and bacteria on the surfaces of the sterilization load; after a suitable exposure period, the sterilant gas is removed from the chamber through the vacuum pump; air is vented back into the chamber to return the system to atmospheric pressure; finally, the sterilized sterilization load is removed.
There are many types of gas-phase chemical sterilants that may be used including, hydrogen peroxide, ethylene oxide, ozone, chlorine dioxide, combinations of these sterilants and others. In particular, hydrogen peroxide is a highly effective vapor-chemical sterilant that can be applied at low temperatures allowing temperature sensitive articles to be processed in the load. Note: a gaseous-chemical sterilant and a vapor-chemical sterilant both utilize the sterilant in gas-phase, but a vapor-chemical sterilant could exist with solid and/or liquid phase sterilant at the temperature of the sterilization process, while a gaseous-chemical sterilant is purely in gas phase. The following U.S. patents, incorporated fully herein by reference, describe such a system and sterilization process in more detail: U.S. Pat. No. 8,230,616 issued Jul. 31, 2012 to McLaren et al. and U.S. Pat. No. 8,366,995 issued Feb. 5, 2013 to McLaren et al.
While the sterilization load should be thoroughly cleaned and dried prior to sterilization, sometimes the load will not be entirely dry. A sterilization load may comprise many items including items with lumens or other channels or crevices that are difficult to dry and may contain liquid water.
As noted above, water in the load can be detrimental to chemical gas sterilization in several ways. Issues may arise due to the higher diffusivity of water compared to the sterilant. This is the case for hydrogen peroxide where the water vapor may reach portions of the load before hydrogen peroxide vapor and diminish the effectiveness of the sterilization action. Other issues may arise if the liquid water remains throughout the sterilization process in either liquid or solid form. The water can block the sterilant gas from contacting portions of the load surfaces, whether they are external surfaces, or internal surfaces such as lumens. In fact, at the operating pressures for chemical sterilization, liquid water can freeze with little chance of removal during the sterilization process.
Several methods have been employed to try to detect liquid water in a load, and to try and remove the liquid water from the load prior to sterilization. U.S. Pat. No. 5,317,896 issued Jun. 7, 1994 to Sheth and Upchurch and U.S. Pat. No. 5,482,683 issued Jan. 9, 1996 to Sheth and Upchurch each describe a detection method using the pump-down time from a pressure above the saturation pressure of water to a pressure below the saturation pressure of water. This pump-down time is compared to a reference pump-down time for a dry system. A longer pump-down would indicate the presence of water. U.S. Pat. No. 5,961,922 issued Oct. 5, 1999 to Witte and Eulogio describes an alternative method in which the pressure is monitored during pump-down below 5 Torr. If water is present, the pressure data often increases for small time periods as the water freezes at the surface and bursts or breaks the ice as the pressure decreases. U.S. Pat. No. 8,230,616 issued Jul. 31, 2012 to McLaren et al., discloses a third water detection method that involves a pump-down to a pre-determined pressure, at which time the pump-down is stopped and the pressure rise is monitored. If the rise is below a threshold value, the load is dry. If the rise is above the threshold, additional steps are taken to attempt to dry the load prior to sterilization.
Each of the methods described above have disadvantages. Simply monitoring the pump-down time will give results that are dependent on the effectiveness of the pump and the integrity of the chamber, both of which are subject to change over time. In addition, the load materials may outgas at varying degrees depending on the load. The materials outgas water vapor, and vapors from oils, greases, solvents, and volatile organics most commonly. Monitoring the pressure below 5 Torr may be a better approach. However, not all liquid water distributions will exhibit an increasing pressure during the monitoring period, so loads with liquid water may be missed and the liquid water may freeze. Finally, monitoring the pressure rise and comparing to a threshold value can work, but may give false indications of water in the load, if the load has a large enough amount of outgassing material. This would trigger additional and unnecessary drying procedures.
It would, therefore, be advantageous to provide a method that is capable of determining whether the load contains liquid water even if the load contains material that is outgassing water vapor and that dries the load in a reliable and verifiable way.
It would also be advantageous to provide a method that detects liquid water in a load that is independent of vacuum pump variations and the underlying chamber leak rate and that dries the load in a reliable and verifiable way.
It would further be advantageous to provide a method that is capable of detecting small amounts of liquid water in a load, without freezing the liquid water and that dries the load in a reliable and verifiable way.